1. Field of the Invention
The present invention relates to an optical cable having a secondary sheath for surrounding and protecting a plurality of optical fiber units in which at least one-core optical fiber is mounted in a buffer tube.
2. Description of the Related Art
As various communication services such as bi-directional communications and image communications are propagated more broadly at home and abroad, a demand for optical cables keeps increasing. Referring to FIG. 1, a general optical cable 20 has a plurality of optical fiber units 10 in which 1- to 12-core optical fiber 13 is mounted in a buffer tube 15 and a jelly 11 is selectively filled therein. The plurality of optical fiber units 10 are inserted into and protected by a secondary sheath 16, and the secondary sheath 16 may be filled with waterproof powder or jelly 17 for making it watertight. In addition, the outside of the secondary sheath 16 is selectively wrapped by a waterproof tape or paper 18, and the outermost layer is protected by a primary sheath 19 made of resin.
Among the elements composing the optical cable, the secondary sheath plays a role of protecting the optical fiber units in the sheath and reinforcing tensile force of the cable that is required for its circumstances. Conventionally, the secondary sheath is frequently made of hard resin such as high density polyethylene and polypropylene with excellent mechanical features in consideration of its primary function for protecting the optical fiber units.
However, such hard polymer materials are not easily shed and require experienced skill for shedding, so they take much time for contact of optical fibers during the cable installation process. That is to say, in order to branch off the cable in a midway position, some strands of optical fibers should be taken out after the primary and secondary sheaths are shed. However, the secondary sheath made of hard polymer materials such as high density polyethylene may damage the buffer tube and the optical fiber when being shed, and it is also difficult to make its surface be cut smoothly. In order to shed the secondary sheath, an effective way is to make a scar on a certain region of the secondary sheath and then tear it. However, the hard high density polyethylene has not good tear characteristic due to bad initial brittleness and strong tenacity. In addition, since the secondary sheath is elongated while being torn, it is not easy to tear it over several ten centimeters smoothly.
Meanwhile, a plasticized PVC made by adding a plasticizer to give flexibility to hard PVC is sometimes used as a material of the secondary sheath. However, the plasticized PVC may cause a fatal problem on the optical fiber in the buffer tube when the contained plasticizer is leaked out, so very limited kinds of materials may be used for the buffer tube.